A ring field optical system is one which provides an annular zone centered about the optical axis wherein images formed in the annular zone are optically well corrected. Ring field systems have found important use in the field of microlithography wherein circuit patterns on a mask in the object plane may be projected without distortion onto the sensitized surface of a silicon wafer in the image plane. Ring field systems are disclosed in general by Scott in U.S. Pat. No. 3,821,763. Ring field systems of the type used in microlithography are disclosed by Offner in U.S. Pat. Nos. 3,748,015 and 4,293,186. All three patents are assigned to the assignee of the present application, and the disclosures of which are incorporated by reference herein. Both of the patents used in microlithography disclose an off-axis annular field optical system for forming in accurate micro detail an image of an object at unit magnification with high resolution. Each employs the concept of convex and concave mirrors in face-to-face relationship with their centers of curvature being more or less concentric. Both systems and particularly the latter were designed to function over a relatively wide radiation bandwidth, i.e., each system remains reasonably well corrected over a wide spectral band.
As microlithography has developed more and more circuits are being packed into each integrated circuit. This requires that the lines of the circuit patterns used in forming such circuits be exceedingly small, i.e. of the order of one micron or less. As is well known this requires use of the lower end of the UV spectrum since it is at this end that image resolution or sharpness is greatest. Recognizing this the industry is now utilizing nearly monochromatic light in the deep UV as the radiation source in mask projection systems. Such sources may be provided by a single line of a mercury lamp or lasers such as the excimer laser.
The present invention relates to a ring field optical system designed to function with such monochromatic radiation sources. In addition, it provides demagnification of the object at the image plane.